The astronomical perspective

Two black holes colliding and merging is a very dramatic event .....…

The astronomical perspective

When Professor Graham Woan joined the team hunting gravitational waves, he estimates that 75% of the LIGO Scientific Collaboration (LSC) could have been classified as “instrumental scientists” – designing, building and operating the detectors – while only 25% were data analysts, developing novel computer-based methods to identify the signals in the midst of the noise. Seventeen years later, he says, the proportions are reversed, and most of the researchers now focus on the use of computers and advanced mathematical methods to interpret the data. At the same time, the roles of the different researchers have also evolved, overlapping in virtually every activity, including the design of the next generation of detectors.

Unlike most other members of the team, Woan started as a radio astronomer who did his first degree in Natural Sciences and his PhD in radio astronomy at Clare College, Cambridge and served his “apprenticeship” at the Mullard Radio Astronomy Observatory and the Cavendish Laboratory. Woan therefore brought a different set of skills to the table when he joined the team in 2000, but since then, the project has changed out of all recognition, not just using bigger and better detectors, but also much more powerful computers, taking advantage of tools such as Bayesian inference methods – which update probability as more data becomes available.

In the early days, Woan’s contribution was not only his theoretical background, but also his practical knowledge of computers, electronics and astronomy. “Radio astronomy attracts people with a broad range of skills,” he says, “and statistics is particularly important for radio imaging.” The pictures of radio sources were fairly crude in the early days of radio astronomy, but clever techniques helped radio astronomers make improvements by using incomplete information on the overall structure of the image to infer the brightness of individual pixels, and build a more complete and detailed image. And similar techniques, says Woan, are used to interpret the signals generated by the LIGO detectors – a kind of “educated guesswork” based on data and experience.

Convergence

Computing and astronomy have gradually come closer and closer together, and Woan believes that Bayesian methods have also become much more relevant because of technological advances, and the vast amounts of data that result.

Woan came to Glasgow in 1996 to join his wife, who already had a job in the English Language department, and he was offered a Lectureship in astronomy and astrophysics, doing research in solar winds, pulsars and interplanetary scintillation (IPS). He was already aware of the work being done in gravitational research, in Glasgow and beyond, and realised he had a contribution to make, providing “astronomical input” to a group populated at that time by specialists in optics and quantum optics, as well as delicate mechanical systems. Gradually, however, Woan expanded his role within LIGO (the Laser Interferometer Gravitational-wave Observatory) as the different groups came closer together to collaborate on Advanced LIGO.

Like most other scientists, Woan believed binary neutron stars would be the first source of gravitational waves to be detected, and they are still a “beguiling source,” he says. Black hole coalescence was also a promising candidate, but a big surprise when that was the first source detected by LIGO.

Woan maintains his interest in continuous gravitational waves, emitted by rotating neutron stars, because these will also provide very valuable data. For example, he keeps a close eye on the Crab Nebula, a supernova remnant in the Taurus constellation which exploded about 1,000 years ago, with a neutron star right in the centre, about 20km across and rotating 30 times a second. “It’s a like a huge flywheel,” says Woan, “and as it throws off lots of energy as electromagnetic radiation, it slows down at a rate of one less rotation in each consecutive 14-hour interval.” If all that energy was released instead as a gravitational wave, Woan explains, we would have detected it several years ago, but we now know that less than one per cent of this energy is emitted in gravitational waves, so the hunt for more data continues. “There are many other good candidates,” Woan says, “including other young pulsars, but also many neutron stars that are not seen as radio pulsars.”

Two black holes colliding and merging is a very dramatic event, but if we saw two neutron stars colliding, we would also learn a lot about what these stars are made of, and answer lots of questions in particle physics. The sensitivity of the detectors is steadily increasing, and the team reduce the noise floor every year, but it may be some time before Woan and his colleagues detect continuous waves.

The ratio of instrumental physicists to data analysts and astronomers may have been turned on its head, but Woan also feels very strongly that data analysts will always have a big role to play in designing the detectors of the future, and that instrumental physicists will always play a big role in data analysis. But no matter where they come from, they all speak the same astronomical language – even if the vocabulary still has words missing.

Biography

Professor Graham Woan leads the data analysis section in the Institute for Gravitational Research (IGR) and has held key leadership roles within the LIGO data analysis working groups for more than ten years, including co-Chair of the LSC/Virgo "pulsar group" (or Continuous Waves Investigations Group). He pioneered the development of Bayesian inference methods in the project and is also a leading authority in gravitational-wave data analysis and astrophysics, leading the efforts to detect gravitational waves from isolated rotating neutron stars known as pulsars – which may be the next type of source that Advanced LIGO detects.